This invention relates to a process for the purification of unrefined sugar solutions. More specifically, it relates to a process for removing turbidity, color, flavor, and odor from impure sugar solutions which may or may not be subjected to further crystallization.
The prior art is primarily concerned with the purification of either raw cane juice or raw sugar syrups, each of which are subsequently crystallized to yield raw sugar (mill sugar) and refined sugar, respectively, leaving in the water phase a great many of the impurities.
In the manufacture of raw sugar (mill sugar), the dark colored raw cane juice, containing gums, waxes, proteins, organic acids, minerals, and particles of vegetable material, is first treated by adding lime to the hot juice. The lime reacts with the organic acids in the juice and forms an insoluble floc with various colloids and with the phosphates in the juice. The floc, containing many impurities, is usually allowed to settle to the bottom of the reaction vessel. Alternatively, the floc may be flotated, in which case more lime and phosphoric acid, or any one of a number of soluble phosphate salts well known to those skilled in the art, may be added to increase the amount of floc. See "Cane Sugar Handbook", Meade and Chen, John Wiley and Sons, New York, 1977, p. 129, and references therein, each of which references is herein incorporated by reference. Polyelectrolytes are usually added to increase the size of the floc particles, and this "secondary" floc is then conventionally flotated by aeration, employing nozzle injection systems, high speed pumping or agitation. The floc-flotation-clarified cane juice is then evaporated in a multi-effect vacuum evaporator and crystallized in a vacuum pan. The mixture of sugar crystals and sugar syrup, or massecuite, is subsequently centrifuged to remove most of the dark mother liquor, or molasses, from the crystals. Residual molasses remaining on the crystals may then be removed with a water spray during continuing centrifugation. The greater the volume of wash water used to wash the crystals, the purer the resulting mill sugar will be. On the other hand, the more water is used, the more the sugar crystals will dissolve, thus reducing the yield of mill sugar. However, no matter how well the crystals are washed, they will most likely contain impurities occluded within the crystals. These impurities could be reduced by using more lime and phosphoric acid in the flocculation step, but again, at the expense of yield. Thus, even the best raw sugars (mill sugars) are likely to contain various impurities.
Such mill sugars are suitable for many products such as candy, bakery products, or as sweeteners for coffee or tea, but not for soft drinks, where the color, aroma, flavor, and turbidity of the sugar may affect the character of the soft drinks and shorten their shelf life. Many sugar mills produce so-called "sulfitated" sugars, wherein the cane juice is treated with sulfur dioxide prior to evaporation. These sugars often have a white appearance, which makes them suitable for certain uses, even though the aroma, flavor, and turbidity may not have been significantly reduced by this treatment. Mill sugar made with other special processing steps, such as extra washing, is called "mill white" or "plantation white," and is also suitable for certain uses. Generally, however, neither sulfitated sugar nor plantation white sugars are pure enough for use in soft drinks, in which higher quality refined sugars are necessary.
In manufacturing refined sugar, crystalline raw sugar, containing a number of undesirable nonsugar constituents, is first washed with water to remove any adhering syrup. The syrup that is washed off the crystals is similar in nature to raw cane juice and is treated separately to recover the sugar from it. The washed sugar is dissolved in water, and the resulting syrup is then clarified by flocculation. The clarified syrup is then decolorized with activated carbon, bone char or other appropriate decolorizing substances to give a purified "fine liquor." The fine liquor is then crystallized to yield refined sugar. The degree of refinement depends on the number, and effectiveness, of the flocculation and decolorization steps.
The flocculation steps usually involve addition of lime and a phosphate ion source, such as phosphoric acid, to the liquor to form a calcium phosphate floc. This floc is conventionally removed by air flotation, often with the addition of a polyelectrolyte in order to form a so-called secondary floc and thereby increase the size of the floc particles. Decolorization of the clarified liquor is usually accomplished by passing it through columns of bone char before the final crystallization.
Soft drink manufacturers virtually always require refined sugar for use in their beverages. However, many countries do not have sufficient refining capacity, and in such countries only mill sugars may be available to certain industries. Before using such mill sugars for soft drinks it is necessary to remove from them the turbidity, color, flavor, and odor, and this must be done in the bottling plant itself.
Various methods for in-plant purification of mill sugar are in use around the world. The simplest method is to prepare a mill sugar syrup and to filter the syrup cold with a small amount of filter aid through a filter press. Such filtration only removes insoluble matter, and does not decolorize or remove flavor and odor. Treatment of hot sugar syrup with activated carbon and subsequent filtration with filter aid is another method currently in use. This process removes turbidity, flavor, aroma, and much of the color, depending on the amount of carbon used. The process has found much favor in bottling plants, but when the sugars are very impure, very large amounts of carbon and filter aid are required in order to bring the sugar syrup up to acceptable purity standards. In addition, the more carbon and filter aid are used, the slower the rates of filtration, and this reduction in production rates may interfere with plant schedules.
Another in-plant purification process presently used includes continuously centrifuging hot syrup to remove turbidity, and subsequently passing the syrup through granulated carbon columns to remove color, odor, and flavor. However, centrifuging is relatively inefficient for turbidity removal, and the degree of color removal in such columns is often not very efficient because the carbon used must be rather coarse in order to allow a sufficiently rapid flow through the column. In addition, because of its gradual loss of decolorization ability, the carbon must be replaced periodically. Still other purification processes involving ion exchange columns are used in some large bottling plants. The ion exchange resins are extremely efficient in removing color, but only if all turbidity has previously been removed, e.g., by filtration with a filter aid. In addition, the ion exchange process is not efficient in removing odor and flavor.
Purification processes involving entrapment of sugar impurities in a chemical floc and subsequent removal of the floc are in use in sugar mills and in refineries, but, almost never in bottling plants.
The floc clarification process consists of adding to the dissolved, usually hot, sugar syrup small amounts of lime and phosphoric acid, or lime and soluble phosphate salts or aluminum sulfate. At about neutral pH the lime and phosphate or aluminum sulfate form an insoluble, calcium phosphate or aluminum hydroxide floc (primary floc) which contains insoluble matter, some of the colloids, and much of the color. The floc cannot be conveniently filtered because of its gelatinous nature. It will settle if given enough time, but it does not compact well enough to obtain a satisfactory yield of clarified syrup. Centrifuging in a continuous centrifuge is not satisfactory either, especially at high sugar concentrations (50.degree.-60.degree. Brix), probably because the turbulence in the centrifuge breaks the floc particles into smaller particles of a density about the same as or less than the syrup, so that a significant amount of floc is left in the syrup after centrifugation. An aluminum hydroxide floc is significantly more difficult to remove from high Brix syrups by centrifuging than calcium phosphate floc.
The most efficient method for removing the primary calcium phosphate or aluminum hydroxide floc is by flotation with a gas. Usually, with either type of floc, a polyelectrolyte is added to form a more easily flotatable secondary floc. The aluminum hydroxide floc is less preferred for use in flotation in high Brix syrups than phosphate floc because it rises more slowly, forms a looser scum, and leaves some floc in suspension. It is possible, however, to obtain satisfactory flotation in high Brix syrups with aluminum hydroxide floc at the expense of very high sugar losses. Thus, aluminum hydroxide is generally not suitable for use in clarification of high Brix sugar syrups. However, aluminum hydroxide may be effectively flotated to clarify low Brix syrups (up to 30.degree. Brix) even at room temperatures.
Flotation of the secondary floc is accomplished by aeration, either by vigorous agitation or aeration with powerful, high shear centrifugal pumps equipped with air inlets. Mechanical aeration, although suitable for refineries, is less suited for bottling plants because of the capital expense of the extra equipment needed and the added utility costs. Further, the aeration step for an average batch may take 15-120 minutes to complete, depending on the size of the pump, thus intefering with plant schedules.
Floc flotation depends on gas bubbles adhering to the floc, and it is known that bubbles of air adhere rather strongly to the primary flocs of calcium phosphate or aluminum hydroxide, and even more strongly to their secondary flocs (produced through the use of a polyelectrolyte). On the other hand, bubbles of oxygen gas, which can be easily generated from hydrogen peroxide by catalase, do not adhere to either the primary or secondary flocs, making the use of oxygen bubbles in flotation seemingly impossible.
It would be of great benefit, therefore, to develop a process for floc flotation which would avoid the capital expense and power costs associated with conventional aeration, and which would overcome the seeming impossibility of using enzymatically generated oxygen gas bubbles to flotate sugar syrup flocs.